The present invention relates to charged particle beam apparatus and more particularly to a charged particle beam apparatus suitable for obtaining a high-resolution image stably by correcting a shift or misalignment of the optical axis of a charged particle beam optical system with high accuracies.
In the charged particle beam apparatus typified by a scanning electron microscope, a thinly focused charged particle beam is scanned on a specimen to obtain desired information therefrom (for example, a specimen image). When, in this type of charged particle beam apparatus, the optical axis deviates in relation to a lens, the lens undergoes an aberration and the resolution of a specimen image is degraded. Therefore, for the sake of obtaining a specimen image of high resolution, a highly accurate adjustment or alignment of axis is needed. JP-A-2002-352758 (Patent Document 1) and JP-A-2003-22771 (Patent Document 2) disclose techniques for making an axis adjustment automatically.
In general axis adjustment, a deflector (aligner) for axis alignment is adjusted so that the amount of a movement of a specimen image, occurring when a condition of an optical element such as an objective lens is changed, can be minimized. Contrarily, the techniques disclosed in Patent Documents 1 and 2 describe a method according to which the deflection condition of the alignment deflector and the state of the optical element as well are changed to obtain specimen images, an amount of a shift between the thus obtained specimen images is calculated under individual conditions and the shift amount is applied to an equation to determine an operational condition of the alignment deflector. The method as above can facilitate the optical axis adjustment even when optical conditions are changed or when the state of a charged particle beam changes as the optical axis changes with time, thus realizing highly accurate auto optical axis alignment or self-optical axis alignment.
Even in the method disclosed in Patent Documents 1 and 2 capable of realizing the self-optical axis alignment, there is a need of further promoting the accuracy. When the deflection condition of alignment deflector and the optical element state are both changed, an obtained image is deteriorated in quality suffering from, for example, a blur and a positional shift amount cannot sometimes be calculated accurately.
The relation between a shift of alignment condition (misalignment) and an image blur will be explained with reference to FIG. 8. When the objective current value for focus control is swung under condition that the alignment condition is undisturbed and the optical axis is thoroughly adjusted or aligned, the beam diameter changes as shown at 801 in FIG. 8. The in-focus state takes place with the beam diameter minimized. In this case, by swinging the object current value by +ΔF and −ΔF from the value for in-focus state, the beam diameter assumes A at both the swung values and images are blurred substantially equally. As a result, images picked up at +ΔF and −ΔF are substantially identical in quality and a positional shift between the two images can be detected correctly.
On the contrary, the state changes as shown at 802 with the alignment condition disturbed. In such an event, the beam diameters at −ΔF and +ΔF are B and C, respectively, differing greatly from each other. In other words, two images blur extremely differently, causing a large difference in their picture quality, and the accuracy of detection of a positional shift between the two images is deteriorated.